Conventional tapping screws are generally classified by basic materials to which screws are coupled for tapping, i.e. materials of objects. In other words, conventional tapping screws cannot be commonly used if materials of objects (injection molded materials (hereinafter, injection-molded objects) such as synthetic resins and the like, light alloy materials such as Al or the like, and steel materials such as thin SECC or the like) are different.
In general, since injection-molded objects, such as synthetic resins and the like, are softer than steel materials, a tapping screw suitable for injection-molded objects typically has a large pitch and a conical shape which ends in a point at the bottom of a thread body, in order to minimize deformation of the injection-molded objects. Since engagement intervals of threads are large in a tapping process due to the large pitch, deformation of the injection-molded objects is minimized and reuse is permitted. Forming the thread body in a conical shape also provides the above-mentioned advantages. Application to an extremely short coupling structure can be hard due to a large pitch (generally, recommended in a structure in which a coupling depth is more than double the length of a screw), lifespan of a mold for screws can be reduced by forming a circle-shaped raw material into a cone-shaped screw, and loss can be caused by preparing different types of molds for different products.
Further, a tapping screw suitable for coupling to soft or hard metallic objects has a smaller pitch than a tapping screw for injection-molded objects. In general, the soft or hard metallic object is formed with a larger hole than the injection-molded objects so as to achieve tapping by applying a certain force (assembly torque value) to threads. A suitable hole determines suitable assembly torque. Since application of the tapping operation to soft or hard metallic objects is relatively difficult due to harder properties thereof than injection-molded objects such as synthetic resins and the like, if a hole is formed similar to that of the injection-molded objects, a torque value is considerably increased and thus an optimal assembly torque value cannot be obtained. Accordingly, a suitable size hole, which is larger than that of the injection-molded objects, decreases thread engagement and results in reduction of torque in a tapping process. However, since decrease in thread engagement can cause unexpected release due to external shock such as vibration or the like, the tapping screw has a smaller pitch and more threads than that for injection-molded objects in order to compensate for this problem. Further, in many cases, tapping screws for soft or hard metallic objects are coupled to short coupling structures, e.g., ultra-thin structures of electronic products, and thus the pitch is smaller than that of the tapping screws for injection-molded objects such as synthetic resins and the like.
As such, the largest difference between the tapping screws for injection-molded objects and the tapping screws for soft or hard metallic objects is the distance of the pitch. If the tapping screws for soft or hard metallic objects are coupled to injection-molded objects such as synthetic resins or the like, the injection-molded objects can be excessively deformed or damaged due to a small pitch, and if the tapping screws for injection-molded objects are coupled to soft or hard metallic objects, the objects can be damaged by lack in the number of engagement threads due to a large pitch.
Due to the aforementioned problems occurring upon application of the tapping operation to different materials, screws cannot be commonly used for various objects made of different materials.
Accordingly, such classification of the screws leads to separate classification of injection-molded object assembly lines and steel product assembly lines, thereby causing various losses.